The present invention pertains to gas turbine engines, and more particularly, to a system and method for better utilizing rotor tip bleed air.
In the past, gas turbine engine pressurized bleed air has been used for a variety of functions which have included turbine cooling, nozzle cooling, purge, pressurization and customer bleed. The source of this pressurized bleed air is most often from the compression system. Rotor blades, in a given stage of a compressor, by performing work upon the air in the air flow path of the compressor, increase the pressure of the airflow. Thus, the downstream pressure in a compressor is greater than the upstream pressure. Stator vanes reduce the tangential momentum in the air received from the adjacent upstream rotor blades so that additional tangential momentum can be imparted by the next downstream rotor at more desirable inlet conditions.
Typically, bleed air is extracted upon exit from a stator vane as the flow path MACH number is lower at this location and the swirl is better controlled. The bleed air can then be piped through proper conduits to a desired user location. The pressure of the bleed air at its source (i.e., the exit point from the stator) must exceed the pressure required at its point of desired use since pressure is lost as air traverses through bends and corners in a conduit.
In a gas turbine engine, the compression process is generally divided into a number of stages. The location of each stator vane in a compressor can be viewed as a possible source of bleed air. Thus, in a given system, it is known what source pressure must exist to achieve a desired pressure at a user location. However, if the source pressure is too high, a system penalty will be introduced as a result of wasted and non-utilized work which of course contributes to engine inefficiency. It is desirable to reduce the number of compression stages to save weight, reduce parts count and lower cost. For the same overall component pressure ratio, the number of possible bleed sources is reduced thus making it more difficult to match the desired level of pressure.
For a hypothetical example, consider the following table:
______________________________________ STAGE N STAGE N + 1 STAGE N + 1 EXIT PRESSURE EXIT PRESSURE RATIO PRESSURE ______________________________________ CASE A 100 1.30 130 CASE B 95 1.40 133 CASE C 110 1.40 154 ______________________________________
Stage N in the above table has an exit pressure of 100 for Case A, an exit pressure of 95 for Case B and an exit pressure of 110 for Case C. The pressure can be viewed as total pressure psi. However, if bleed air, having a source pressure of 120 is required, an exit pressure at stage N is insufficient for Case A, Case B and Case C. In stage N+1, an excess exit pressure exists in Case A (130-120=10), an excess exists in Case B (133-120-13), and an excess exits in Case C (154-120=34). The selection of a stage for supplying bleed air depends on the pressure drop between the stage and zone of use.
In summary, if the pressure is insufficient, it cannot be utilized at all, but if it is too great, penalties result from the excess pressure. This excess pressure is a result of excess work being performed. The pressure in excess of the desired pressure results in wasted work which is a penalty to the system. If additional work is performed on the bleed air, the bleed air is given a higher temperature which necessitates a greater quantity of bleed air for cooling purposes.
Thus, a need exists for a system and method for obtaining a desired bleed air exit pressure to enhance engine efficiency.